The invention concerns a method and a device for the measurement of the volumetric flow of a fluid
A method or device of this kind is known from WO 93/12 405, which describes a refueling system for motor vehicles where the volumetric measuring device consists of a screw spindle counter. The screw spindle counter consists of two intermeshing wormdrive screw spindles, one of which carries a magnet. Rotation of this magnet as a result of rotation of the screw spindle causes the generation of pulse-shaped measurement signals from a sensor element, which co-operates with the magnet, and which are fed to a measuring transducer which converts such measurement signals into counting signals for use by a counter stage. The pulse generator, integrated into the measuring transducer, contains a pulse alignment and pulse shaper stage which allows electronic adjustment of the pulses generated by the rotation of the screw spindles for calibration purposes before these are fed to the electronic counter with its arithmetic unit. In this way, pulses can be modified in amplitude and repetition frequency by external check and control pulses, which are derived from the arithmetic stage of the counter unit, so that they may, for example, be matched to an arithmetic relationship with the volume flow of the fuel uplifted. Within the arithmetic unit of the electronic counter, these check and control pulses may be adjusted by suitably authorized personnel to account for certain operational or climatic conditions or, when required, for calibration purposes.
Another type of electronic flow meter is disclosed in U.S. Pat. No. 4,885,943, employing a turbine, a detector for detecting the passage of turbine blades past the detector due to fluid flow within the turbine. The output from the detector, comprising a series of pulses, is processed to compensate for non-linearity in flow occurring at low flow rates, which is a similar problem to that which occurs with screw spindle meters of the type with which the present invention is concerned and which problem the present invention addresses.
In the case of a screw spindle counter, or a counter in which the displacer consists of paddle wheels, or in the case of counters having an oscillating piston as the movable element, then their construction often results in non-linearity between the test signal and the actual volume flow. For example, in the case of a low flow rate, the fluid quantity associated with one pulse may be greater or smaller than the flow rate associated with one pulse when the flow rate is higher.
The invention is based on the task to improve a generic device, respectively a generic process in respect of measurement accuracy.
The task is solved by the present invention.
According to the invention, signal correction takes place in the transducer, whereby the correction factor sued depends on the flow rate of the medium to be measured. For this purpose, the cycle frequency is determined, as a measure for the flow rate, of the movable element, respectively of the screw spindle. Relating to a plurality of cycle frequency values, corresponding correction factors are stored in a table or similar means within the transducer. Using the appropriate correction factor for the corresponding cycle frequency, the transducer derives counter signals and feeds these to the counter. Preferably, transducer signals are pulse-shaped, but they could also follow a sinusoidal law. Preferably, the cycle frequency is derived from the pulse duration or pulse chopping rate. In a preferred version of the invention, each pulse measured is weighted by the correction factor with a volume level. Volume levels are then dependent on the flow rate. For example, at a flow rate of 10 l/mm, the volume level may be 8 ml, whilst, for constructional reasons, at a flow rate of 1 l/mm, this increases to 11 ml. Such volume levels are summed up by the transducer which then generates a counter signal as soon as the sum total of all volume levels has reached a multiple of a standard volume level. If, in the case of the example mentioned, 10 ml were chosen as the standard volume level, and if refueling takes place at a flow rate of 1 l/min, then the transducer will generate a counter signal with the first measurement signal. If, on the other hand, refueling takes place at a flow rate of 10 l/min, then the transducer will not issue a counter signal after receiving the first measurement signal, since its memory store only holds a value of 8. Only after receipt of the second measurement pulse and when the memory contains a value which reaches or exceeds the standard volume level of 10 ml, a counter signal is generated. It is also conceivable that the value of the memory store is reached by the amount of the standard volume level after generating a counter signal so that counter signals are always issued when the memory store exceeds the level of the set standard. The memory store then only contains the remaining balance. Preferably, correction factors are stored in a table within the transducer for a multitude of cycle frequencies. In addition to the correction factors, a calibration factor may also be stored which, after signal linearisation to generate counter signals, is multiplied with the latter as in the state of the art by means of a proportionality factor so that he actual volume flow, dependent on operational and climatic conditions, may be shown on the display, and respectively may be added up by a counter. Correction factors between two levels contained in the table may be interpolated. Preferably, the transducer contains a microprocessor. Conversion of test signals into counter signals may then be micro-program controlled. A counter may add up the counter signals and the display may be either the sum total or the current flow rate.
An example of a version of the invention is explained by means of the appended schematic representation.